Suddenly, everybody has caught up with the fact that a slowdown is on the horizon. If this perception is as late this time as it has been in the past, we had better start thinking about the economy's faltering, not just slowing.

For some hint as to whether the economy was or was not expanding too rapidly, attention should have been directed away from GNP to other measures. GNP tells us where the economy has been, not where it is going. The ''other'' economic measures or indicators told us that forces were in place and were moving into place that would have slowed the economy further.

Further? Has the economy been slowing down?

Most observers seem to be saying GNP did not rise as fast in second-quarter 1984 as it did in first-quarter 1984. It did slow down, but it still advanced rapidly. A better way to look at what happened was that GNP rose relatively rapidly in second-quarter 1984, but its growth still slowed.

An even better way to look at what has happened is to appreciate that while GNP did rise relatively rapidly in second-quarter 1984 and while GNP did slow its pace of growth in second-quarter 1984, neither development contributes to indicating where the economy is going.

On innumerable occasions, GNP has risen at a rapid rate only to be followed by a slower rate. On other innumerable occasions, GNP has slowed its growth rate only to be followed by a rapid rate.

What GNP did last quarter does not tell us what it will do in the next quarter, or the next, or the next.

At this fairly late stage in an economic upswing - the average upswing unfueled by major increases in military spending has been only 25 months - we are usually warned of a slowing future pace by the performance of so-called leading and lagging economic indicators.

GNP is not one of those indicators. GNP is a coincident indicator, an economic measure whose movements coincide with the direction of the economy as a whole.

Leading indicators tend to change direction before the economy as a whole changes.

Many of these leading indicators are providing signs of having peaked and turned downward. Before their August surge, common-stock prices were down since last October. Average hours worked in manufacturing are down since April; business formation down since February. Credit change shows a high in March; vendor deliveries, in March; sensitive prices change, last August; orders received by manufacturers, in January; initial unemployment claims, February; and housing permits, February.

Of the 12 basic leading indicators, nine of them are below earlier, late 1983 or early 1984 highs. So you see, behind the major gains of GNP, changes in other economic measures have been occurring and they have been downward. It may be difficult to believe, so let us repeat it. These other changes have not simply been smaller gains than those in GNP, they have been decreases.

These decreases should not and cannot be looked at as exceptions. The decreases confirm the role that after 20 months of an economic upswing, the cyclical process usually induces weaknesses in a selected group of economic measures that precede weaknesses in GNP and other broad indicators of the economy as a whole.

Linked to a softening in the leading indicators at this stage of an upswing are pervasive strong gains in so-called lagging indicators. Lagging indicators turn upward or downward after the economy as a whole. As they gain momentum they slow the upswing of the economy.

Included among the basic lagging indicators are the movements of the prime interest rate, up in earnest only since February 1984, 16 months after the economy as a whole started its upswing. Other laggers whose rapid movements inhibit an upswing are the ratio of manufacturing inventories to sales, up since January; commercial and industrial loans, up rapidly since January; and the ratio of installment debt to personal income, accelerating rapidly since January.

To look at recent rapid gains in these laggers as indicative of rapid gains to come in the economy as a whole is to misunderstand their place in the economic process. Rapid gains in interest rates, business and consumer loans, and inventories are signs that GNP has been moving upward too rapidly and may be ready to slow.

The reporting of movements of economic measures with no reference to their characteristic timing relative to the movement of the economy cannot provide a clue to where the economy is going. It is absolutely essential to a coherent understanding of what is developing that one know whether an economic statistic qualifies as a leading, coincident, or lagging indicator.

While we are told that it is vital to keep one's eye on the ball, in assessing the economy it is essential to keep one's eyes on the three balls of leading, coincident, and lagging indicators.

Currently, leading economic indicators are weakening, coincident economic indicators such as GNP are exuding strength, and lagging economic indicators are accelerating their upward pace. This combination of developments has traditionally preceded a slowing of the economy's upward pace. If developments persist for a time, economic upswings subsequently turn into downswings or recessions.

Late in 1982, few perceived the end of recession. In the early months of 1983 , few perceived the strength of the upswing. In the first half of 1984, most everyone exaggerated the strength of the economy; few perceived an immediate slowdown. Today, everyone is talking slowdown. Is this perception any better? Or will these observers be surprised once again when slowdown becomes recession.

Tear your eyes away from GNP and recognize that the economy is undergoing a major change in its behavior. That change suggests that at the very least, the economy is poised to slow its recent upward pace further. At worst, it may be heading for a recession.